The present invention relates to electrically actuated friction devices and more particularly to an electromagnetically actuated self-adjusting clutch.
Electromagnetic actuated clutch devices have been found to be particularly desirable in applications wherein the transmission of power to a user of the power must be controlled and transmitted by clutch device having considerable compactness and longevity. Such an application is exemplified by a compressor in an air conditioning device. Such an application places particularly tough requirements on the clutch in that it must be simple and economically constructed and at the same time transmit a high torque output. In addition, the clutch may be either off or on for long periods of time or frequently cycled on and off over a short period of time depending upon the demands placed upon the air conditioning system in which it operates. Therefore, the clutch must be sufficiently durable to withstand constant use with a minimum amount of downtime for adjustment.
Heretofore, there have existed at least two recognizable types of electromagnetically actuated clutch constructions. One type, herein called the "cone" variation has inter-engaging friction surfaces between engageable rotatable clutch elements which are conically shaped and by their inherent geometrical configuration require a lower axial force to develop sufficient frictional locking of the working faces for rotation of the driven members. The other type, herein called the "disc" variation has inter-engaging friction faces which are generally disposed normal to the axis of the engageable rotatable clutch element and, in instances where the flux path passes through the friction faces, the armature pull is entirely axial. The disc type configuration is particularly advantageous due to its ability to provide a large axially directed flux path and thereby provides for a strong clutch engaging force.
Other prior art designs are known which combine the desirable characteristics of these two types of electromagnetic actuated clutches. One such design provides an armature ring element which is generally L-shaped in radial cross-section; the element has one annular pole piece with a frusto-conical face and another annular pole piece with a flat disc-like face disposed normal to the clutch axis. This design, however, has several drawbacks. For example, since the armature ring element is one piece, there is no means for compensating for wear of either pole piece. Further, the conical friction surface must be made of a magnetic material. In addition, the outer pole force is almost entirely in the radial direction instead of the preferred axial direction.
Another prior art design uses the same principle; threading the conical piece to the disc-like pole piece. This design has the added drawback of forcing the electric flux path through the threads which results in a loss in the generated clutching force produced by the electromagnetic actuating device.
Yet another prior art design such as that shown in U.S. Pat. No. 3,679,034 issued on July 25, 1972 to Miller provides a conical frictional element which is moved into engagement with a mutually engageable conical face on an output member by the use of a resilient torque transmitting member. The resilient torque transmitting member provides a sufficient force to disengage the mutually engageable conical surfaces when the electromagnetic force becomes de-energized. Since the force of the resilient disengaging torque transmission member is in a direction opposite to the direction of the force generated by the electromagnetic flux path, out of necessity it requires a greater electromagnetic force to engage the mutually engageable friction torque transmitting faces. Therefore, as wear occurs, the electromagnetic force required to engage the frictional faces becomes higher in magnitude since the resilient torque transmission has to be deflected a greater distance. Further adjusting for the wear on such prior art device causes adverse wear on the armature face opposite the pole face. This is a result of the centrifugal force acting on the adjusting means. The centrifugal force causes the adjusting means to be somewhat delayed thereby causing the pole face of the output member to come into contact with the armature while rotating for a sufficient duration of time to cause wearing of the armature face.
In some known devices, such as that shown in U.S. Pat. No 3,994,379 issued on Nov. 30, 1976 to Miller and Giometti, the frictional element threadably engages with a concentrically disposed armature thereby providing for axial movement of the frictional element relative to the armature as the frictional element wears. A substantial amount of heat is generated by the frictional element which could cause the threaded engagement to bind because of thermal expansion with the result that the frictional element would no longer freely move relative to the armature.